In particle colliders used in high energy physics, such as the Superconducting Super Collider (SSC), large solid angle detectors are used to provide data of the trajectories of subatomic particles. The large solid angle detectors can be augmented by including a device for performing high precision tracking close to the bunch crossing point (i.e. within a few tens of centimeters of the point where the beams collide). Such a device is called a microvertex detector. The main purpose of a microvertex detector is to provide "close-in" particle track reconstruction with point-to-point spatial resolution of a few microns. A microvertex detector allows precise location of the primary event vertex, of secondary vertices from decays of c and b quarks or .tau.leptons, and of vertices unrelated to the event of interest from other beam-beam interactions. In addition, the event vertex constraint and the track segments close to the interaction point improve the overall tracking momentum resolution provided by the large outer tracking chamber.
Several factors impose severe constraints on the design and construction of a microvertex detector to be used at colliders such as the SSC. These factors include the high charged particle multiplicity within events, the requirement for fast detector response arising from the 16 nanosecond bunch spacing, and the high yearly radiation doses a detector would receive near the beam pipe. A microstrip detector made of silicon would be the conventional choice since it would represent proven technology and would adequately address the first two factors mentioned above. With respect to sensitivity to radiation damage, silicon strip detectors are only marginally acceptable for long term operation at a collider such as the SSC. For example, 1 MRad is about the yearly dose expected at 10 centimeters radially away from the interaction point due to minimum ionizing particles from beam-beam interaction. After a 1 MRad dose of radiation, a silicon strip exhibits a greatly increased leakage current. Other more out-of-the-ordinary detectors such as scintillating fibers or semiconductor pixel devices either exhibit some change in their operating characteristics after MRad doses of radiation or have unknown radiation hardness. Therefore, it would be desirable to have a detector that would show no measureable change in its operating characteristics even after several years operation at the SSC.
There exist superconducting materials that are more resistant to damage by radiation by several orders of magnitude compared to semiconductor devices. The present invention utilizes the radiation hardness exhibited by superconducting materials, such as NbN, in order to fabricate a superconducting thin film strip detector sensitive to minimum ionizing particles for use as a microvertex detector.
The idea of using a superconducting film as a particle detector was first proposed approximately 25 years ago by N. K. Sherman in an article entitled "Super-conducting Nuclear Particle Detector" Phys. Rev. Lett. Volume 8, page 438 (1962). The device was envisioned as a detector of fission fragments or alpha particles. Experimental observation of voltage pulses from super-conducting-to-normal (S-N) transitions caused by alpha particles of about 5 MeV kinetic energy incident on thin indium, tin and aluminum films has been reported in D. E. Spiel, R. W. Boom and E. C. Critterdon, Jr., Appl. Phy. Lett. Volume 7, page 292 (1965); E. C. Crittendon, Jr. and D. E. Spiel, J. Appl. Phys. Volume 42, page 3182 (1971); N. Ishihara, R. Arai, T. Kohriki and N. Ujiie, Japan. J. Appl. Phys. Volume 23, page 735 (1984); and K. W. Shephard, W. Y. Lai and J. E. Mercereau, J. Appl. Phys. Volume 46, page 4664 (1975).
The experiments and papers described above were directed at alpha particle detection. More stringent requirements exist for minimum ionizing particles. Also, the above described experiments did not show how to incorporate the thin film strips behavior into a practical device for use as a microvertex detector in collider experiments. In particular, previous experiments did not account for the requirement for accurate readout of data from a thin film strip.
Therefore, it is an object of this invention to provide a microvertex particle detector for use on minimum ionizing particles in a collider which exhibits high precision with respect to particle detection but is resistant to deterioration by radiation.
It is another object of this invention to provide a means for determining the spatial coordinates of particles impinging a superconducting thin film strip particle detector in a short period of time.
It is still a further object of this invention to provide a superconducting thin film particle detector with means for readout of data which overcomes N.sup.2 ambiguity.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.